Friday, October 27, 2017

Another Problem For Particle Dark Matter Theories

The galactic cluster BCG wobble is essentially the galactic cluster scale analog to the long standing core-cusp problem with Cold Dark Matter at the galaxy scale. The press release accompanying this new paper suggests that self-interacting dark matter is the only solution, but other studies broadly rule out all astrophysically relevant parameters of SIDM models.

A striking signal of dark matter beyond the standard model is the existence of cores in the centre of galaxy clusters. Recent simulations predict that a Brightest Cluster Galaxy (BCG) inside a cored galaxy cluster will exhibit residual wobbling due to previous major mergers, long after the relaxation of the overall cluster. This phenomenon is absent with standard cold dark matter where a cuspy density profile keeps a BCG tightly bound at the centre. We test this hypothesis using cosmological simulations and deep observations of 10 galaxy clusters acting as strong gravitational lenses. Modelling the BCG wobble as a simple harmonic oscillator, we measure the wobble amplitude, Aw, in the BAHAMAS suite of cosmological hydrodynamical simulations, finding an upper limit for the CDM paradigm of Aw<2kpc at the 95% confidence limit. We carry out the same test on the data finding a non-zero amplitude of Aw=11.82+7.3−3.0kpc, with the observations dis-favouring Aw=0 at the 3σ confidence level. This detection of BCG wobbling is evidence for a dark matter core at the heart of galaxy clusters. It also shows that strong lensing models of clusters cannot assume that the BCG is exactly coincident with the large scale halo. While our small sample of galaxy clusters already indicates a non-zero Aw, with larger surveys, e.g. Euclid, we will be able to not only to confirm the effect but also to use it to determine whether or not the wobbling finds its origin in new fundamental physics or astrophysical process.

Using the NASA/ESA Hubble Space Telescope, astronomers have discovered that the brightest galaxies within galaxy clusters "wobble" relative to the cluster's centre of mass. This unexpected result is inconsistent with predictions made by the current standard model of dark matter. . . .

[C]lusters have very dense cores, each containing a massive galaxy called the "brightest cluster galaxy" (BCG). The standard model of dark matter (cold dark matter model) predicts that once a galaxy cluster has returned to a "relaxed" state after experiencing the turbulence of a merging event, the BCG does not move from the cluster's centre. It is held in place by the enormous gravitational influence of dark matter. But now, a team of Swiss, French, and British astronomers have analysed ten galaxy clusters observed with the NASA/ESA Hubble Space Telescope, and found that their BCGs are not fixed at the centre as expected.

The Hubble data indicate that they are "wobbling" around the centre of mass of each cluster long after the galaxy cluster has returned to a relaxed state following a merger. In other words, the centre of the visible parts of each galaxy cluster and the centre of the total mass of the cluster -- including its dark matter halo -- are offset, by as much as 40,000 light-years. "We found that the BCGs wobble around centre of the halos," explains David Harvey, astronomer at EPFL, Switzerland, and lead author of the paper. "This indicates that, rather than a dense region in the centre of the galaxy cluster, as predicted by the cold dark matter model, there is a much shallower central density. This is a striking signal of exotic forms of dark matter right at the heart of galaxy clusters."

The wobbling of the BCGs could only be analysed as the galaxy clusters studied also act as gravitational lenses. . . . This effect, called strong gravitational lensing, can be used to make a map of the dark matter associated with the cluster, enabling astronomers to work out the exact position of the centre of mass and then measure the offset of the BCG from this centre.

If this "wobbling" is not an unknown astrophysical phenomenon and in fact the result of the behaviour of dark matter, then it is inconsistent with the standard model of dark matter and can only be explained if dark matter particles can interact with each other -- a strong contradiction to the current understanding of dark matter.

"Would it be correct to say that simple models of dark matter have virtually been ruled out, but more complex models of a dark sector have not?"

Yes and no. A lot of simple models of dark matter have been ruled out. But, in the early days of dark matter modeling, a lot of studies were done of mixed dark matter models that were more complex, but the simpler models were almost always a better fit to the data than the mixed dark matter models, so a lot of more complex models aren't part of the discussion because they were among the first to be ruled out. The key points are that any viable dark matter model needs both some non-gravitational self-interaction and some non-gravitational interaction with baryonic matter, although they could be the same thing. Neither of them can be Standard Model forces (but neither the weak force nor the strong force act at distances sufficient to provide the interaction with baryonic matter, and EM interactions by dark matter are strongly ruled out - otherwise it wouldn't be dark).

"Also would it be correct to say that the models of galaxy formation/evolution using MOND rely on a pretty complex model of baryonic matter? For example papers like this include star formation in the model in order to reproduce, and I would think that would require knowledge of all three Standard Model forces, correct?"

Not really.

First of all modified gravity theories, because they are designed from the get go to mimic dark matter phenomena actually behave very similarly to dark matter models at the cosmology scale, although the analysis that gets you there is very different.

Secondly, dark matter models need a very specific model of galaxy formation/evolution to get the relative distributions of baryonic matter and dark matter in relation to each other to distribute themselves just so. But, in modified gravity theories, you automatically get dark matter phenomena equivalent to that very precise distribution of dark matter as a function of whatever baryonic matter distribution you have, no matter how the baryonic matter is distributed, so you get all of the dark matter distributions for free as a function of your baryonic matter distributions instead of having to have two mostly independent functions end up in a very specific baryonic Tully-Fisher relationship, for example.

"Would it be fair to say that the debate is between either a simple modification of 1 force or a complex dark matter model, but with no real indication of whether the universe "should" be simple?"

The universe "should" be simple.

For example, you can model all kinetic dynamics cause by gravity and dark matter phenomena combined in toy model MOND from Earth scale and solar system scale to the scale of the largest elliptical galaxies with a single free parameter. So, non-toy model modified gravity or dark matter theories should also have only one dominant degree of freedom over that immense range of scale. You might need one to three additional degrees of freedom to get all of the way up to galactic cluster scales and dark energy phenomena, but the "true" theory really shouldn't need more degrees of freedom than the simplest toy model modified gravity models that can describe the same phenomena, even if the toy models themselves are actually flawed for some reason or other (e.g. predicting gravity waves of the wrong speed or not being relativistic period), in a way that a final correct theory cannot be.

It shouldn't take nine parameters to model something that toy models come very close to getting right with two to four parameters.

Deur's work suggests that it might actually be possible to model all gravitational, all dark matter and all dark energy phenomena with just a single experimentally measured coupling constant for the graviton, although as a practical matter, it might be convenient to have several physical constants associated with gravity just as we have both Newton's constant and the constant for the strength of the gravitational field on the surface of the Earth that scientists and engineers memorize and use on a day to day basis. Deur's work gets the complexity of gravitational phenomena in significant part from the three dimensional distribution of baryons in space without actually having to incur another experimentally measured fundamental physical constant.

A graviton based theory of gravity probably means that you need a beta function for the coupling constant of gravity, but the constants in a beta function aren't experimentally measured, they are determined some very complicated math instead (and a graviton based theory of gravity also tweaks all of the beta functions of all of the other parameters in the Standard Model slightly at high energies, again in a non-experimentally measured mathematically calculable manner, possibly even leading to gauge unification which the SM comes close to but never hits using beta functions not adjusted to reflect gravitational impacts on those functions).

But, the beta function is probably almost entirely irrelevant outside systems that are singularities in GR (i.e. the Big Bang and Black Holes), something that the Post-Newtonian approximation's unexpected accuracy also suggests. So, unless you are doing Big Bang cosmology or trying to come up with theory about the event horizon and interior of a black hole, or trying to develop a TOE, the beta function of quantum gravity can be safely disregarded.

"I would think that would require knowledge of all three Standard Model forces, correct?"

You can pretty much entirely dispense with the weak force except to get some basic neutrino parameters, and neutrino effects are small enough to be disregarded in lots of models (including lambdaCDM).

You can ignore the strong force in all parts of the model except for baryogenesis after the Big Bang and in stars. For the most part EM and a periodic table of the elements model with nuclear interactions in stars treated like black box phenomena that produce particular outputs on particular time scales for particular stellar masses, are all that you need in your model of galaxy/structure formation in addition to gravity, and for lots of purposes, assuming that everything is made out of hydrogen and helium is a good first order approximation that captures all young stars and gas giants, but omits the spectral features of older stars and rocky planets and asteroids and moons and planetoids all of which collectively make up a very small percentage of all baryonic matter and don't meaningfully impact large scale structure or galaxy formation. If you make heavy element fractions in stars a phenomenological function of star age, you can do even better with minimal additional complexity. And, honestly, the gravitational effects pretty much swamp the EM effects just as they do the neutrino effects in large scale structure/galaxy and cluster formation models once you get past the radiation era in the first half-billion or so years after the Big Bang.

So, bottom line, you can do very well with a model that includes hydrogen, helium, gravity and a phenomenological function describing star composition and supernova effects based upon star age, while otherwise ignoring all three SM forces to the extent not captured in this simplification. Omitting dark matter makes it significantly easier to model that a dark matter plus baryonic matter model.

@Andrew - " For the most part EM and a periodic table of the elements model with nuclear interactions in stars treated like black box phenomena that produce particular outputs on particular time scales for particular stellar masses"

This is exactly my point though - how the hell would you even know you need this black box for the dark sector to begin with? Suppose there's a dark sector that has its own equivalent standard model forces, but that doesn't couple to any of the actual standard model forces. How would you even know that there are supernova to account for in the first place based on gravity data alone?

"And, honestly, the gravitational effects pretty much swamp the EM effects just as they do the neutrino effects in large scale structure/galaxy and cluster formation models once you get past the radiation era in the first half-billion or so years after the Big Bang."

Source? You're talking about a higher level of structure than I am, but I'd be very surprised if radiative cooling is irrelevant past 0.5 billion years.

"The universe "should" be simple. "

Says who? Why does the universe have to obey our sense of aesthetics?

"It shouldn't take nine parameters to model something that toy models come very close to getting right with two to four parameters."

Aren't there 18 parameters for the Standard Model? Does that make the Standard Model wrong? Suppose there is a Dark Sector - why would it have to be any simpler than the Standard Model?

"The key points are that any viable dark matter model needs both some non-gravitational self-interaction and some non-gravitational interaction with baryonic matter, although they could be the same thing."

That doesn't seem to be the case for all models - they don't all actually need some non-gravitational interaction with baryonic matter.

"Source? You're talking about a higher level of structure than I am, but I'd be very surprised if radiative cooling is irrelevant past 0.5 billion years."

For example, in the Planck experiment report on the fitting of the lamdaCDM model and possible expansions of it, EM interactions do very little to tweak lamdaCDM predictions.

"Says who? Why does the universe have to obey our sense of aesthetics?"

I'm not saying "should" in the aesthetic or hopeful sense. Indeed, I am a rather fervent proponent of avoiding beauty as a quality for evaluating physical theories.

Instead, I'm saying that if a system whose data can be almost completely explained by a small number of parameters in any model as an empirical matter, that the underlying true description is probably fairly simple if even the correct model is a different one. My evaluation is driven by the patterns in the data, not by my personal preference regarding what a theory should look like.

"Aren't there 18 parameters for the Standard Model? Does that make the Standard Model wrong?"

The Standard Model has so many parameters because you need that many to describe the data (although there is reason to think that one could devise a "within the Standard Model" theory that would turn some of them into derived quantities). There is no alternative to the Standard Model that uses fewer parameters.

I'm really simply applying a cousin of Occam's Razor. The true theory is unlikely to have many more free parameters than the most parsimonious model you can come up with that comes very close to explaining the data.

The issue is not the number of parameters in an absolute sense. I could perfectly well imagine a gravity theory that had 30 parameters. But, if you can find another theory that predicts equally close to reality results with four parameters, the true explanation probably doesn't need 30 parameters.

"That doesn't seem to be the case for all models - they don't all actually need some non-gravitational interaction with baryonic matter."

One of the breakthrough finds of 2017 by at least two independent researchers, is that you simply cannot explain the distribution of dark matter that is observed without interaction with baryonic matter.

Also all astrophysically relevant SIDM theories have been excluded.

The 2014 paper you cite is outdated.

Some of the relevant citations collected in this recent blog post: http://dispatchesfromturtleisland.blogspot.com/2017/10/dark-matter-particle-theories-are.html

@Andrew - "I'm really simply applying a cousin of Occam's Razor. The true theory is unlikely to have many more free parameters than the most parsimonious model you can come up with that comes very close to explaining the data."

I realize. I'm just questioning whether or not Occam's Razor is applicable here.

"For example, in the Planck experiment report on the fitting of the lamdaCDM model and possible expansions of it, EM interactions do very little to tweak lamdaCDM predictions."

That's talking about a very different era and a very different scale than what I talked about though. lamdaCDM works just fine does it not?

Thanks. Looking at the Salucci paper, I'm having a hard time seeing what's so conclusive about it. They talk about the close correlation between inferred DM and luminous matter - which I agree would be good evidence for MOND though not 100% conclusive - but I don't see where they rule out gravitational interactions as the source of this? It seems to be mostly an assumed point no?

Salucci is pretty definitive about ruling out unmodified GR interactions as the source of this. Indeed, they frame the issue in their introduction as follows:

"We associate, as usual, the huge local mass discrepancy in galaxies with the presence of surrounding halos made by a massive elementary particle that lays outside the HEP Standard Model (e. g.: see Bertone et al. (2010)). This particle also does not interact significantly with atoms, photons and with itself, through strong, weak and electromagnetic force. This does not strictly require that DMP must interact with the rest of the Universe only through gravitational force, but that, such eventual interaction must be much weaker with respect to the ordinary baryonic matter vs baryonic matter interaction. Moreover, no current observation prevents the existence of interactions between the dark and the luminous sector of elementary particles that result relevant in the galaxy formation context. However, so far, the simplest dark matter scenario has been routinely adopted, according to which the DM halos are made by WIMP particles, more precisely by collisionless cold dark massive particles that interact very feebly with themselves and atoms."

The conclusion goes on to state:

"we claim that the structure of the inner parts of the galaxies is driven by a direct interaction between Dark and Luminous components. The DM central cusp, foreseen for any heavy collisionless DM dark matter particle and also in many other cases, with an increase of DM pressure at lower radius, gets, as time goes by, progressively eaten up/absorbed by the dominant luminous component. The interaction flattens the density of DM and drops the pressure towards the center of the galaxy."

(1) you get NFW density distributions (something computed analytical long ago) which aren't what is inferred, (2) you get very different halos in spiral galaxies with and without large central bulges, which isn't what is observed, (3) you get far fewer spiral galaxies without bulges than are observed, (4) you get a spherical distribution of satellite galaxies rather than a plane distribution that is observed, (5) you have M/L ratios that aren't a function of galaxy scale as is observed, (6) you would have higher proportions of DM in elliptical galaxies and lower proportions in dwarf galaxies which is the opposite of what is observed,(7) you get lower velocities in colliding clusters than what we see, (8) you get higher rotational velocities in dwarf satellite galaxies near the Milky Way and don't get an external field effect, (9) you get slower structure formation in the early immediate post-radiation era universe,(10) you get more scatter in rotational velocity relative to galaxy mass,(11) you don't get distributions of DM that are fine tuned to the particular distribution of baryonic mass within the system, and(12) you don't get a halo fitting rule that works equally well in dwarf, spiral and elliptical galaxies - each should have a different fitting rule.